Optical waveguides are structures that constrain or guide the propagation of light along a path defined by the physical construction of the waveguide. The dimensions of the waveguide in the direction in which the light is confined are on the order of the wavelength of the light. Such optical waveguides comprise a region of high refractive index in which most of the optical field of the light is located surrounded by regions of lower refractive index. Typically, an optical waveguide comprises a three-layer or sandwich structure comprising a substrate, a middle or film layer, and a top layer or cover. The top layer or cover is very frequently air. The index of refraction is largest in the middle or film layer and the light is actually guided in this layer.
Optical waveguides may be constructed so as to confine the propagated light in either one or two dimensions. Optical waveguides which confine light in only a single dimension comprise a film simply laid out as a planar layer atop a substrate. Optical waveguides that confine light in two dimensions rather than one, typically comprise a film laid out as a pattern of channels or strips on the substrate. The width and depth of the channels are on the order of the wavelength of the light to be guided, thus confining the propagation of light along the channels.
Channel waveguides have been made in the past by a variety of different techniques. One of the simplest and most effective methods known for making channel waveguides in glass is by the technique known as ion exchange. According to this technique, a base glass containing, e.g., sodium ions is covered with a metal mask. The glass may typically be an alkali aluminoborosilicate or a soda-lime glass. The metal mask covers the surface of the base glass except for the places where narrow channels are desired. The base glass covered with the mask is then immersed in a molten salt bath. In some cases, an electric field is applied to the molten salt bath. The molten salt bath consists a source of single valence ions, such as alkali metal, thallium or silver ions, which diffuse into the glass in the uncovered regions and replace the sodium ions at the glass surface. This results in a pattern of channels in the glass wherein the channels have higher density and altered electronic polarizability compared to surrounding regions. Both of these effects lead to a higher index of refraction, and thus to ion- exchanged channel waveguides in the glass substrate.
Another technique which has been used in the past to form channel waveguides is photolithography. According to this well-known technique, a suitable optical material is dissolved in a solvent, spin-coated onto a substrate, and exposed to ultraviolet light through a photomask. In a typical negative photolithographic process, the ultraviolet light causes the exposed portions of optical material to polymerize and harden. After this, the unpolymerized portions of the optical material are washed away to form the waveguide channels. In a typical positive photolithographic process, the ultraviolet light causes the exposed portions of the optical material to decompose while the unexposed portions harden. The decomposed portions are then washed away. In either case, a pattern of raised strips or channels of the optical material is left behind on the substrate.
Photolightographic techniques are not entirely satisfactory for fabricating channel waveguides. In the first place, such techniques may leave rough edges on the channels, which can cause unacceptably high losses of light intensity. In the second place, unless special methods are adopted, photolithographic techniques are not suitable for fabricating optical waveguides from organic optical materials for reasons discussed below.
Recently, there has been increased interest in the use of organic optical materials for optical devices such as optical waveguides. In Auston et al., "Research on Nonlinear Optical Materials: An Assessment", 26 Applied Optics, pp. 211-234 (1987), which is incorporated herein by reference, a review is presented on recent research into optical materials, including organic and polymeric materials, and their use in optical devices. Many of these organic materials have highly desirable electro-optic or nonlinear properties. The conjugated polymers in particular, such as the polyacetylenes and polydiacetylenes, are known to have high third-order optical nonlinearities. These organic optical materials will be referred to herein as nonlinear organic or "NLO" materials. Not only do these NLO materials exhibit large optical nonlinearities, but they also have ultrafast (on the order of femtoseconds) response times. Thus, they appear to be suitable candidates for all-optical devices requiring only modest input or control power. Single-mode channel waveguides in particular can exploit the optical properties of NLO materials since the optical confinement provided by these structures results in a very high intensity of light being propagated over macroscopic distances without appreciable losses in intensity.
Heretofore, it has been difficult to fabricate low-loss, single mode channel waveguides from NLO materials for a variety of reasons. In the case of the conjugated polymers, such as the polyacetylenes and polydiacetylenes, the intractability of the substances has been an impediment to their use in the manufacture of optical devices. Only recently have several soluble polydiacetylenes been synthesized and described in the literature. See, e.g. Miller et al., 185 Makromol. Chem (1984), at p. 1727, and DE-OS 3347618 which are incorporated herein by reference. However, even with the advent of suitable solvents for NLO materials, another significant stumbling block to the manufacture of channel waveguides from NLO materials in the incompatibility of most NLO materials with known photolithographic techniques due to the high absorption of light by the NLO materials.
More specifically, typical channel waveguides are about 0.8-2 microns thick, most preferably about 1.5 microns thick. At thickness of about 1.5 microns, light losses are low during the propagation of the 1.3 to 1.67 micron light upon which most optical communication networks are based. However, because most NLO materials absorb ultraviolet light very strongly, the ultraviolet light which is used in photolithographic processes will not penetrate into the NLO layer more than about 0.4 microns in depth. It is therefore not possible to form channel waveguides with depths greater than about 0.4 microns out of NLO materials by standard photolighographic techniques.
In U.S. patent application Ser. No. 176,647 filed for G. L. Baker and C. F. Klausner, on Apr. 1, 1988 and assigned to the assignee hereof, a novel photolithographic technique is disclosed for manufacturing channel waveguides from polydiacetylenes. The technique disclosed therein involves several steps including the steps of applying more than one layer to a substrate and developing an image in one of the layers.
Other techniques which have been used in the past to manufacture single mode and multimode channel waveguides from NLO materials include growing nonlinear organic crystals in glass capillaries of various sizes (see, B. K. Nayer, "Optical Second harmonic Generation in Crystal-Core Fibers," OSA Digest of the 6th Topical Meetion on Integrated and Guided Wave Optics (1982)), and growing such crystals in the grooves of a glass substrate (see, S. Tomaru et al., Optics Commun., 50, 154 (1984)). However, these are difficult and cumbersome fabrication techniques.
Accordingly, it is an object of the present invention to provide a channel waveguide structure made from an optically nonlinear organic material.
It is a further object of the present invention to provide a channel waveguide structure made from NLO materials which can function, if desired, as a low-loss, single-mode or multimode channel waveguide at wavelengths of interest such as wavelengths in the range of about 1.3 to 1.67 microns.
It is another object of the present invention to provide a novel method for fabricating such a channel waveguide structure which method does not suffer from the deficiences of prior art techniques.
It is yet another object of the present invention to provide a simple method for fabricating a channel waveguide structure that does not employ photolithographic techniques.